A family of ferroelectric-ferromagnetic composite materials are described in U.S. Ser. No. 08/265,899 filed Jun. 27, 1994, now U.S. Pat. No. 5,512,196 which disclosure in its entirety is incorporated herein by reference. Such composite materials are particularly useful in electronic filtering elements to suppress electromagnetic interference (EMI). The electromagnetic interference filters find particular application in an automotive environment where there is an abundance of stray radio frequency noise, interference between electrical devices, and noise created by the making and breaking of circuits, spark discharges, poor contact between metal bonds and components, and atmospheric interference. Such EMI sources in an automobile pose a serious threat to the electrical integrity of electrical circuitry and the function of electrical components.
Suitable ferroelectric materials for the subject ferroelectric-ferromagnetic composites include barium titanate, barium strontium titanate, barium strontium niobate and barium copper tantalate. Typically, barium titanate is the preferred ferroelectric material because it has a high permittivity (.epsilon./.epsilon..sub.0) of about 1000 or higher at about 1 kHz. The ferromagnetic constituent of the composite is a ferrite which is a high electrical resistance magnetic material consisting principally of ferric oxide (Fe.sub.2 O.sub.3) and one or more other oxides. Suitable ferromagnetic materials are certain of the AB.sub.2 O.sub.4 -type, ferromagnetic ferrites where A is at least one element selected from the group consisting of copper, magnesium, zinc, nickel and manganese and B is iron. A particularly preferred ferrite because of its electromagnetic properties and its relatively low sintering temperature is a copper-zinc-magnesium containing, AB.sub.2 O.sub.4 type ferrite with excess A component. (Cu.sub.0.2 Mg.sub.0.4 Zn.sub.0.5 Fe.sub.2 O.sub.4) is exemplary of such suitable copper-based ferrites. In general, both the ferroelectric constituent and ferromagnetic constituent are employed as very fine grained particles, suitably about one to five microns in diameter, and they are thoroughly mixed together and compacted and sintered to form a composite body in which grains of each material are percolated or interconnected with each other, i.e., each grain of ferroelectric has neighboring grains of ferromagnetic and vice versa. However, the grains of ferroelectric and ferromagnetic are not chemically reacted. The distinct phases of the ferroelectric and ferromagnetic granular constituents essentially retain their distinct properties.
For many electromagnetic filtering applications, it is preferred to employ approximately equal parts by volume of the ferroelectric fine grained constituent and the ferromagnetic fine grained constituent. When, e.g., barium titanate is employed in combination with the copper-zinc-magnesium ferrite, such an equal volume ratio mixture is compacted and sintered at a temperature of the order of 1185.degree. C. Such a sintered composite provides excellent electromagnetic attenuation or filtering properties, especially at frequencies of 100 MHz to 1 GHz, as more fully described in the above-identified U.S. patent application. However, in many such filtering applications, it is desirable to form a device of multiple layers of the subject composite material with internal metallic electrodes positioned between the refractory oxide layers. High temperature firing, such as at 1185.degree. C., limits the choice of electrode materials to expensive precious metal conductors such as a gold-platinum-palladium ternary alloy. It is desirable to retain the benefits of the composite sintered ferroelectric-ferromagnetic bodies while processing them by firing at a temperature of the order of 1100.degree. C. so that the energy costs of sintering can be reduced and a lower cost electrode material can be employed. If the sintering temperature of the barium titanate/magnesium-copper-zinc ferrite composites (or of other family member composites) could be reduced to about 1100.degree. C., then, e.g., a silver-palladium electrode material consisting of about 70% by weight silver and 30% by weight palladium could be employed at considerable cost savings.